Fighting Gram-positive bacteria with African medicinal plants: Cut-off values for the classification of the activity of natural products

Abstract

The present chapter focused on the antibacterial potency of African medicinal plants and their constituents against drug-sensitive and drug-resistant Gram-positive bacteria. Based on collected data, we have established rationale cut-off values for the classification of antibacterial agents towards the Gram-positive bacteria. For botanicals: Outstanding activity: minimal inhibitory concentration (MIC) ≤ 8 µg/mL; Excellent activity: 8 < MIC ≤ 40 µg/mL; Very good activity: 40 < MIC ≤ 128 µg/mL; Good activity: 128 < MIC ≤ 320 µg/mL; Average activity: 320 < MIC ≤ 625 µg/mL; Weak activity: 625 < MIC ≤ 1024 µg/mL; Not active: MIC values> 1024 µg/mL; (ii) For phytochemicals: Outstanding activity: MIC ≤ 2 µg/mL; Excellent activity: 2 < MIC ≤ 4 µg/mL; Very good activity: 4 < MIC ≤ 8 µg/mL; Good activity: 8 < MIC ≤ 32 µg/mL; Average activity: 32 < MIC ≤ 64 µg/mL; Weak activity: 64 < MIC ≤ 512 µg/mL; Not active: MIC> 512 µg/mL. Based on the above cut-off points, we have identified five African plants displaying outstanding to excellent activities on Gram-positive bacteria. They include Macaranga capensis Benth. (Euphorbiaceae); Macaranga kilimandscharica Pax. and Macaranga conglomerata Brenan (Euphorbiaceae), Salvia africana-lutea L. (Lamiaceae), Erythrophleum lasianthum Corbishley (Caesalpinioideae). Compounds with the corresponding inhibitory effects were plumbagin, emodin, and rapanone. These botanicals and phytochemicals deserve further in-depth studies to develop new medicine to combat Gram-positive bacteria.